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Rhizocephalans of the suborder Kentrogonida are parasitic barnacles whose female cypris larvae must locate a suitable host organism (a decapod crustacean), while the male cypris larvae must find a host containing a virgin female. Some of the sensory structures assumed to be involved in this behaviour are the aesthetasc-like setae found on antennules of the cyprids, of which the females possess one and the males two. To verify the aesthetasc nature of these setae we examined their morphology and innervation in the cyprids of the “model” kentrogonid rhizocephalan,Sacculina carcini, using SEM and TEM. No structural differences were found between the two types of aesthetasc-like setae, and their ultrastructure strongly indicates them as being aesthetascs. They have an exceedingly delicate cuticle, 20-50 nm thick, and the lumen contained essentially only very fine ciliary branches of which many were in close contact with the cuticle. No sheath cells were seen in the setal lumen. The ciliary branches could be traced back through the fourth and third antennular segment. The bodies of the sensory cells were not located, but morphological evidence suggests that their inner dendritic segments are located in the second antennular segment, giving rise to cilia that evolve into sheath-wrapped ciliary bundles. All these characteristics support their role as aesthetascs and olfactory organs, while the absence of these traits in the so-called terminal aesthetasc invalidates its previous description as an aesthetasc altogether, although it may still be a chemosensory seta. [ABSTRACT FROM AUTHOR]

Nauplii of the rhizocephalan families Peltogastridae and Lernaeodiscidae carry a torus-shaped collar around the body. It consists of an exceedingly thin cuticle connected to the general body cuticle along a continuous narrow ridge. In nauplii of some species, the collar is very large and its surface ornamented by a very conspicuous reticulated pattern of ridges. In other species the collar is smaller and with a smooth surface that impedes its detection when using a light microscope. The collar is absent from nauplii of all investigated species of the Sacculinidae. Transmission electron microscopy shows that the collar of the succeeding nauplius instar is formed in an unexpanded state beneath the old cuticle and it must therefore be inflated at or immediately after ecdysis. At ecdysis the collar of the old instar breaks along the attachment ridge, leaving the empty collar and the exuvium of the general body as separate objects. The collar must have a profound influence on the hydrodynamic properties of the nauplius, both when swimming and passively sinking. We therefore consider it as a “floatation device”, a view supported by the absence of the collar in the rapidly swimming cypris larvae. There is no obvious homology to the collar outside the Rhizocephala, and it therefore appears to have evolved only once. [ABSTRACT FROM AUTHOR]

Provides information on a study which examined the vermiform stage in the parasitic barnacles called Rhizocephala, a crustacea. Materials and methods; Observations on Loxothylacus panopaei and Loxothylacus texanus; Morphology of the vermigon; Discussion.